1. Field of the Invention
This invention relates in general to a device for identifying an individual by identification of his or her fingerprint(s). This invention relates in particular to a fingerprint sensor for transforming the information contained in a person's fingerprint into an electric output signal.
2. Description of the Prior Art
Fingerprint identification systems which identify the print of a finger pressed on a contact surface are well known in the art.
U.S. Pat. No. 4,053,228, for instance, discloses a finger identification apparatus containing a transparent glass plate which serves as a contact surface or fingerprint reader. A fingerprint is formed by pressing a finger against the back surface of the glass plate and holding it in a predetermined position thereon. The fingerprint is interrogated by a light beam directed through the front surface of the glass plate. The interrogating beam is partially reflected at the back surface to provide a signal beam carrying the fingerprint information. The reflected signal beam is correlated against a hologram of the same fingerprint to provide the identification.
In U.S. Pat. No. 4,120,585, another fingerprint identification system is disclosed. This system contains a pliable optical prism as a fingerprint reader. The base of the prism is physically contacted by the finger of the person under investigation. The pliable prism deforms under the applied pressure. It partially reflects a sensing light beam to a photosensitive device which is activated. The photosensitive device, in turn, activates further optical components of the fingerprint identification system. Also in this case, a fingerprint reader is used which is examined for the ridge-valley pattern of a fingerprint of the person to be identified.
In the two U.S. Patents mentioned above, the fingerprint is formed either on a glass plate or on the base of a glass prism. The fingerprint is instantly investigated by irradiation of light. Neither mechanical nor electrical storage of the fingerprint is provided. Yet, it is advisable to read the information of the fingerprint pattern into a computer. The computer then can make an investigation with regard to specific features and with regard to the identification according to some stored information. Therefore, there is a need to store the fingerprint information of a person in a computer. That requires a fingerprint sensor for transforming the fingerprint information into an electric output signal which can be fed into a computer.
The article "Piezoelectric and Pyroelectric Polymer Sensors", Proc. Conf. on Sensor Technology for Battlefield and Physical Security Applications, Fort Belvoir, VA, July 13-15, 1977, pp. 204-212 (U.S. Army Mobility Equipment Research and Development Command, Fort Belvoir, VA, July 1977) discloses synthetic organic polymers having piezoelectric properties. Piezoelectric polymers are commercially available in thin layers. They are compliant, flexible, tough, light, and relatively cheap. They can easily be attached to a surface. Rubber cement, cyanoacrylate, epoxy, or other cements can be used. One of the commonly used piezoelectric polymers is polyvinylidene fluoride (PVDF). The piezoelectric modulus of this polymer is about six times as large as for a typical piezoelectric ceramic, such as lead zirconate titanate ceramic. A piezoelectric polymer obtains its strong piezoelectric characteristics by heating the material, applying a strong electric field, and returning to room temperature with the electric field applied. Such a process results in the alignment of a significant number of electric dipoles normal to the plane of the sheet. Any stimulus that changes the thickness of the sheet will change the surface charge density on each surface. A more detailed analysis has been published in J. Appl. Phys. 46,4204 (1975). Either the charge, a short circuit current or an open circuit voltage can be measured to determine, for instance, the pressure on a sheet of piezoelectric polymer. The flexibility, light weight, and freedom from fatigue of polymers make them suitable for measuring various physical parameters, including pressures. The thinness and flexibility of polymer gauges make them feel and act mechanically very much like skin. Therefore, polymer gauges can be applied like band-aids to monitor, for instance, heart sounds and pulse rates of patients during exercise. Arrays of gauges can be used for acoustical holography. The active portion of a gauge can be cut to any reasonable size and shape needed for the measurement.
According to the paper "Piezoelectric and Pyroelectric Polymers", supra, and to U.S. Pat. No. 3,970,862, a typical sensor for measuring temperature or pressure consists of a sandwich of two thin polymer sheets. Each sheet has evaporated metal electrodes on both faces, and the sheets are cemented together so that charges of the same polarity appear on the inner faces. The center conductor of a coaxial cable is connected to the electrodes on these inner faces, and the shield of the cable is connected to the electrodes on the outer faces. In this way, all exposed surfaces are at ground potential, and the signal inside the sensor is well shielded. Yet, sensors for measuring the distribution of pressure are not disclosed in the J. Appl. Phys. article.
In the Technical Support Package entitled "Transducer With A Sense of Touch" of JET Propulsion Laboratory, California Institute of Technology, Pasadena, California, November, 1979, is disclosed a touch or pressure sensor which determines the shape and pressure distribution of an object in contact with its surface. The sensor outputs can be displayed as an array of alphanumeric symbols on a video monitor, or they can be used to develop a pressure "map" of the surface of the object. The signals can also control mechanical or electrical equipment. The touch sensor consists of a matrix of small electrodes in a metal frame overlaid with a sheet of pressure-conductive plastic. The frame which consists of many cells is held at ground potential, and a common power source is applied to the electrodes inserted into each cell. Pressure on the plastic sheet varies the conductance of the path between an electrode and the metal frame. Thus, the current flowing through the electrode generates a voltage measure of pressure across a resistor in series with the electrode. The voltages in the matrix convey information about the shape and surface contours of the object which contacts the plastic sheet. The disclosed transducer is not a fingerprint sensor in the sense of the present invention. Fingerprints shall neither be detected nor evaluated. The object is that a Mechanical Hand will be fitted with a touch sensor on one or more fingers.